It is well known to protect iron and steel structural elements against corrosion by a suitable paint or coating. Structural elements made of iron and steel and submerged in water, particularly hulls of ships and the like, frequently are protected not only by a paint or similar coating, that is, by a passive corrosion protection, but additionally by an active corrosion protection by subjecting the steel plate to an electrical voltage. Cathodic corrosion protection involves connecting the steel object which is to be protected as a cathode with respect to one or more anodes and applying a protective current thereto, to maintain the article to be protected at a predetermined negative protective voltage. A suitable range is in the order of about -950 to -1150 mV, measured with respect to an Ag/AgCl cell.
It has been found that customary paint coatings and similar coatings applied on steel surfaces as a protection against corrosion are damaged by the additional cathodic electrical loading if a combination of paint protection and cathodic protection is desired. Customary paint coatings, upon being cathodically loaded, are prone to form bubbles. In one coating which has recently become available to be applied to steel surfaces, a current density in the order of about 50 mA/m.sup.2 is required in order to maintain a suitable protective potential thereon. As the age of the paint coating increases, the current density to maintain the protective potential rises, and may reach values in the order of 100 mA/m.sup.2 and thereover. As the current density increases, the tendency to form bubbles also increases.